Method for testing bituminous paving mixtures



May 29, 1923. 1,457,015

F. s. BESSON METHOD FOR TESTING BITUMINOUS PAVING MIXTURES Filed Aug. 5. 1921 Patented May 29, 1923.

mn'rnon roe TESTING BITUMINOUS PAVING MIXTURES.

Application filed August 5, 1921'. Serial No. 490,172.

(FILED UNDER THE ACT OF MARCH 3, 18 83, 22 STAT. L, 8 25.)

To all whom. it may concern:

Be it known that I, FRANK S. Bussoiv, a.

citizen of the United States, and a resident of \Vashington, District of Columbia, have invented an Improvement in Methods for Testing Bituminous Paving Mixtures, of which the following is a specification.

The invention described herein may be used by the Government, or any of its ofiicers or employees in prosecution of work for the Government, or by any other person in the United States, without payment to me of any royalty thereon.

This invention relates to the production of paving mixtures, more especially it is directed to a new and improved method for rapidly and accurately determining the worth of an untried aggregate, based on known conditions and results.

One of the principal objects of this invention is to provide a method of the character designated which will enable anaccurate diagnosis of bituminous paving mixtures in advance of laying and permit of a uniform mode of procedure, affording definite dependable data relative to the durability and probable life of such mixturesunder trafiic conditions.

Another important object of this invention is to provide a method, through the instrumentality of which it becomes possible to accurately ascertain the character of mineral aggregate best suited to the purpose for which the mixture is to be used.)

Another object of this invention is to provide a method of mixing bituminous pavings which will enable the use of aggregates locally available and eliminate the necessity of employing artificial mixtures which in some parts of the country, by virtue of their cost, are almost prohibitive.

Other objects of this invention are to eliminate so far as it is possible the factors giving rise to defective bituminous paving mixtures, and to place the theory of pavin design upon a practical basis.

lVith these and other objects in view this invention consists in certain novel details of procedure and proportioning of materials as will be more specifically hereinafter set forth and claimed.

In the companion a plication entitled Bituminous paving, ed May 26, 1920, Serial No. 472,887, there is described a method by which the bituminous binding agent and mineral aggregate are proportioned and combined in accordancewith a previously arranged diagram to effect what may be termed a normal mix. This diagram, while affording accurate and satisfactory results in respect to the amount of bitumen required for any particular, aggregate, fails to impart information concerning the character of the aggregate to be used, or the worth of the resulting mixture for specific purposes.

In order to definitely determine these problems, it has been necessary to supplement the data initially obtained with additional developments to the end, that the haphazard and indefinite mix and try? methods heretofore employed may be replaced with an accurate mode of procedure which will, to a large extent, standardize the production of bituminous pavings.

In bituminous surfaces, embodying binder and topping, there are three elements to which failure may be attributed: '(a) binder course, (b) bitumen in the topping,

(c) mineral aggregate, but which of these should be blamed for defective paving mixtures has always been a matter of conjecture.

As a result of investigations, it has been found the question of binder can best be eliminated by omission of this course, especially as the surface without the binder is but one half the usual thickness and would therefore require much less material ,for'

maintenance.

As to the second factor ofuncertainty,

that is to say, bitumen in the topping, which involves both the character of the bitumen and the amount to be employed with any particular aggregate, this question has now been largely determined by accurate specifications guaranteeing uniform bitumen products, and the diagram method disclosed in the application hereinbefore referred to.

In regard to the third factor productive of defective paving mixtures, namely, mineral aggregate, it has been found that when:

such trafiic as a guide in the preparation of future mixtures. Manifestly this is a highlv unsatisfactory procedure, owing to time required, to say nothin of the expense involved, and it is there ore the aim and purpose of this invention to provide a means whereby tests can 'be quickly made in the laboratory which will provide reliable in formation as to both the character of aggregate, the probable life of the mixture and the usage to which it is best suited.

In the accompanying drawing in which corresponding facts areindicated by similar reference characters,-

Figure 1, is an elevation of one form ofi testing apparatus, with a test-block operatively positioned, and I Figure 2, is a sectionized detail partly in elevation, illustrating the assembly of guide, clamp and test-block.

Briefly stated, this improved method con sists in selecting a series of samples from various streets which have long been subjected to extreme traiiic conditions, forming these samples into blocks of a predetermined size, subjecting each 'block to the penetration of a pin at a known temperature, the pin being actuated by a weight falling through a given distance, recording the number of blows required to either crack or penetrate the sample, the nature of the .fractures, and utilizing this data as a basis in the diagnosis of original mixtures.

In actual practice, cylindrical, compressed blocks, 10 c.m. in diameter, are made of the mixture to be tested, each block containing 1,000 grams of mineral aggregate and as many grams of bituminous cement as may be found necessary to give the required percentage of bitumen. A steel pin 19 mm. in diameter is centered over the block and subjected to a series of blows delivered by a miniature pile driver weighing 20 kilograms and falling through a distance of .8 meters.

Although various forms of ap aratus may be employed in carrying out the tests forming the subject matter of this application, the design found to afiord'the most satisfactory results is shown in the" accompanying drawings and embraces a base 1), parallel guides (2) and 3), and cross-head (4). Between guides (2 and (3) is slidably positioned a 20 kilogram weight (5 suspended by a block and tackle {6). an (7 Weight 5 is releasably secured to the tackle by means of a trip (8), consisting of a bill portion (9) and an extended leg portion (10). The bill portion (9) normally engages an eyelet (11) secured to the weight (5), while the leg portion (10) cooperates with a contact piece (12) located on the guide (3) to effect disengagement of bill portion (9) when the weight has attained the desired elevation.

In operation a cylindrical specimen (A) is properly positioned on support (1) and upon said specimen is disposed a pin guide (1'3) secured in position by a clamp (14). This guide is formed with a central aperture (15) in which is seated a pin (16) adapted to be driven into the specimen by the fallin; weight for the purpose hereinbefore alleged.

)Vith the use of the a paratus just defined after a number 0 blows, the pin either completely fractures the block, or perforates it to its full available length, 60 mm., accompanied by more or less cracking. A record of the blows delivered, their general effect, together with a close study of the fractured blocks, will either suggest the use of similar mixtures or warn against them.

In the application of this improved method. photographic records are made of the results of the tests and used as laboratory models; but for the purpose of this application these results have been set forth in tabu lated form as will hereinafter appear.

To obtain suitable models of specimens for laboratory determinations,samples were selectedfrom various streets in and about the District of Columbia, formed into blocks as previously stated, and tested. The results of the tests are as follows:

N 0. Street descriptions. Blows. Result.

1 Economical service ren- 5 to 7 at The block specimens dered over a number of 77 F. break into 2 or 3 years. N 0 noticeable pieces with clean defects. planes of fracture.

2 Contain a network of l to 3 at Specimens break into cracks,dividingthesur- 77 F. three or more large face into patches, each pieces with more or containing an area of a less crumbling. sq. yd. or more. Give better service in summer than in winter.

3 Extreme old age accoml to 3 at Specimens break into anied by some crack- 77 F. three or more large ng. ieces with more or ess crumbling.

4 The surface pushed into 1 to 4 at Specimens give verywaves or bumps. Un- 77 F. ing results and at 77 comfortable to ride over. F. little inference can Give better service in be drawn. See No. winter than in summer. 5-Tests at 140 F.

5 The surface pushed into i or 2 at The pin penetrates to waves or bumps. Un- 140 F. its full depth with comfortable to ride over. little or no crackin'g Give better service in of the specimens. winter than in summer.

6 Contain a network of 3 to 5 at The specimens break cracks, dividing the sur- 140 F as do those from good face into patches, each streets described in containing an area of a N o. 1 when tested at sq. yd. or more. Give 77 F. better service in summer than in winter.

With reference to the preceding tabulation, the first specimen represents a sample of a mixture taken from a pavement that may be placed among the oldest serviceable sheet asphalts in this country. The fact that it is a most excellent mixture has been proved by a life of 42 years, its service rendered to-day being more economical than that of most surfaces but half the age.

Comparing samples 1, 2, 3 and 4, while the difference between good and cracked pavings becomes instantly apparent, considerable difiiculty was at first experienced in distinguishing between the mixtures which would result in cracked pavements and those which shoved into waves, inasmuch as both, at a temperature of 77 F., gave substantially the same results as shown by specimens 2 and 4 of the above list.

To solve this problem, a study of the waved streets was made whereupon it was noted that maintenance during the hot months was heavier than in the cooler ones, and that the waves iron out in the winter to reappear in the summer. The idea was thus obtained that these tests should be made at a temperature higher than the usual room temperature, accordingly a thermometer was buried in a street and a temperature of 140 F. noted. Tests were then made at this temperature, and the difference between mixtures resulting in cracked or wavy pavements became immediately manifest as evidenced in. specimens 5 and 6 of the preceding tabulation. In the case of mixtures producing a wavy pavement at a temperature of 140, the in penetrated the block to its full de th, w ile with mixtures resulting in cracke pavements the block broke somewhat similar to good specimens tested at average temperatures. This coincident is in accord, however, with the fact that though badly cracked streets generally show much disintegration during the winter, they render fair service in the summer.

While a comparison of the results between tests made at 77 F. and 140 F., establishes a clear line of demarkation between cracked and wavy pavements, it is more expedient to collect specimens from three classes of pavements (a) good, (b) cracked, (0) wavy; form five blocks from each specimen and then test the blocks of the respective specimens at 32, 50, 80, 110,

and'140'F., a comparison of the results of these tests will supply suflicient data for satisfactory conclusions concerning the different mixtures.

In view of the above it will be seen with the aid of these laboratory samples it is possible to make up new mixtures and readily diagnose the possible worth of such mixtures by comparison with such models. In this connectionv it may be said, in order to have the blocks on a strictly comparative balsis, the same bituminous cement is used in al The necessity of subjecting bituminous cements to rigid tests has long been recognized, but heretofore no tests have been commonly applied to determine the relative suitability of mixtures as effected by the mineral matter employed. With the aid of this block test, however, it becomes possible to readily ascertain the influence of dust upon an aggregate, and the effect of changing the quantity of bitumen, the susceptibility of aggregates to temperature changes and the action of aggregates as influenced by the consistency of bitumen predetermined by the penetration test. These determinations will be considered and illustratedin the order mentioned.

Variations in behavior of different aggregates when mixed with a percentage of bitumen other than normal, is illustrated in the following tabulation showing the influence of dust upon an aggregate and the effect of changing the quantity of bitumen, test being made at 77 F., the asphalt having a penetration of 50 units.

Description of aggregate.

Too little bitumen.

Normal bitumen.

Too much bitumen.

100% s a n d passin the No. Omes Well graded. 47.1% voids.

85% No. 10 sand with limestone dust. 45.6% voids.

70) No. 10 sand tan limestone dust.

47.1% voids.

No. 10 sand. 5071lmes t on e d u s t. 50.7% voids.

30% No. 10 sand. 70%limes t o n e 56.6% voids.

100% s t o n e voids.

lime- 8.3% bitumen. 3 blows breaks the specimen in twowithslight crumbling.

9.1% bitumen. 4 blows b r e a k s t h e s p e c1 m e n in two with slight crumbling.

12.3% bitumen. 7 blows b r e a k s t h e s p e c l m e n cleanly in two.

13.4% bitumen. 9 blows b r e a k s t h e s p e c 1 m e n cleanly in two.

17.4% bitumen. 4 blows breaks the specimen cleanly in two.

12.3% bitumen. 6 blows breaks the specimen cleanly two.

11.4% bitumen. 5 blows breaks the specimen cleanly in two.

12.3% bitumen. 8 blows b r e a k s t h e s p e ci m e n cleanly in two.

15%bitumen. 6 blows. The pin penetrates with no crackmg.

15.6% bitumen. 6 blows. The pin enetrates wi cracking.

20% bitumen. fiblows. The pin netrates wit no cracking.

15.2% bitumen. With 8 blows the pin lly p e n e trates with one radial crack.

14.6% bitumen. With 8 blows the pin fully enetrates with one radial crack.

15.2% bitumen. 6 blows. The pin penetrates with no cracking of the specimen.

17.4% bitumen. 5 blows. The-pin penetrates with no cracking of the specimen.

18%bitumen. 4 blows. The pill. penetrates with no cracking of the specimen 22.5% bitumen. 4blows. The pin enetrates wi no cracking of the specimen.

In the light of the above test, it is evident that bituminous requirements depend upon the voidage and not on the presence or absence of certain sizes in the mineral matter, as has lon been assumed.

Because of t e known influence of temperature chan es on a bituminous paving, it is essential t at the mixture be as insensitive to temperature as possible. When conducting investigations as to the character of mixture best suited to the conditions to which it is to be subjected, it is necessary to make a temperature test of the sample.

As an illustration, a test has been made using a No. 10 sand having a voidage of 44.6% the bitumen percentage being 11, and the asphalt penetration units, the result being as follows:

Tempera- Result.

ture.

32 F. 2 blows. The specimen breaks cleanly into three. 50 2 blows. The specimen breaks cleanly into two. 70 3 blows. The specimen breaks cleanly into three. 90 6 blows. About one-third of the specimen breaks ofi. One

radial crack in the larger portion. 100 8 blows. About onethird of the specimen breaks ofi. One

radial crack in the larger portion. 130 6 blows. Tlie pin penetrates. Several radial cracks showng. 150 5 blows. The pin penetrates. Several radial cracks showmg. 180 3 blows. The pin penetrates. Several radial cracks showing.

With reference to the above tabulation, it

is to be noted when heated to more than 100 F the mixture becomes soft and is penetrated by the pin thus indicating that in pavings subject to high temperature, such a mixture would produce a wavy street.

In the foregoing, the effect of the addition of dust in the mixture and the influence of temperature changes has been discussed and shown. It now remains to illustrate the action of aggregates as influenced by the consistency of bitumen predetermined by the penetration test. Accordingly, a test has been made using a No. 10 sand with 45.6

voids having a bitumen requirement of 11.5.'

The results of this test are as follows:

With the aid of this test it is evident for the mixture used, a cement having a penetration of about 50 units affords the best results.

In conclusion it may be said that successful experiments have conclusively demonstrated that the herein described method 'for designing bituminous paving mixtures will materially reduce the cost of conactuated by a body falling through a given distance, recording the number of blows required to cause the element to become imbedded in the block or to pulverize or crack it, similarly treating a like formed block of original mixture and comparing the number and effect of-the blows on the latter with those on the first mentioned block to determine the probable service value of the original mixture.

2. A method for testing bituminous paving mixtures consisting in selecting a paving specimen of known service value and composition, forming it into a block of predetermined form and weight, subjecting the block, at a definite temperature, to the impact of an element actuated by a body falling throu h a given distance recording the number 0 blows required to cause the element to become imbedded in the block or to pulverize or crack it, similarly treating a like formed block of original mixture and comparing the number and effect of the blows on the latter with those on the first mentioned block to determine the probable service value of the original mixture.

3. A method for testing bituminous paving mixtures consisting in selecting a paving specimen of known service value and composition, forming it into a series of blocks, each having a redetermined form and weight, subjecting t e respective blocks, at temperatures ranging from 32 to 140, to the impact of an element actuated by a body falling through a given distance, recording the number of blows required to cause the element to become imbedded in the block or to pulverize or crack it, similarly treating a series of like formed blocks of original mixture and comparing the number and efi'ect of the blows on the latter with those on the first mentioned blocks to determine the probable service value of the original mixture.

FRANK S. BESSON. 

